Make upgrades less manifest ordered by numbered script prefixes work.
Allow a single script upgrade without manifest; prefix is optional.
Ensure a missing prefix raises an exception.
Have scripts_from_*() generate the path package directory from the version.

The simplification became apparent when calling scripts_from_upgradedir()
in a test which required passing it a full path to an upgrade package
directory when the object has ther base directory as a property.
Add test for invalid prefixes.